This invention relates to a process for the treatment of mixtures of platinum group metals, gold and silver, and is more particularly concerned with the treatment of so-called platinum group metal concentrates such as matte leach residues and the like, and also various intermediate solid products such as leach insolubles etc. which may arise during the processing of such materials.
All commercial refining methods for the platinum group metals are basically hydrometallurgical in nature. Thus the first and key operation in these processes is the dissolution of the valuable components from the source material.
However, the platinum group metals are extremely difficult to dissolve, and extended leaching periods with highly active media such as aqua regia are commonly used. Even under very vigorous leach conditions complete dissolution of the values is not generally achieved, and in particular the secondary platinum metals (rhodium, ruthenium, iridium and osmium) are attacked to only a small extent. In addition the degree to which the metals dissolve is often highly variable from batch to batch and is generally dependent on the nature of the process steps to which the source material has been subjected prior to the platinum metal leaching step.
Because of these effects it is common practice to use a multi-stage process involving both pyrometallurgical and hydrometallurgical steps to effect dissolution of the platinum metals. One commonly used series of operations is as follows:
(1) performing an initial aqua regia leach on a concentrate of platinum group metals, gold and silver; PA0 (2) smelting the residue with litharge and a reductant to form a lead-platinum group metal alloy; PA0 (3) parting of the lead bullion so formed with nitric acid; PA0 (4) leaching the residue from step (3) with aqua regia which dissolves most of the platinum and palladium but little of the secondary platinum group metals; PA0 (5) fusing the residue which is rich in secondary platinum group metals with sodium peroxide; and PA0 (6) leaching of the fusion mixture with water and neutralising with hydrochloric acid to produce a chloride solution of the secondary platinum group metals. PA0 (1) a process whereby a large proportion of the base metal content of the feed material can be effectively leached from the precious metal content before the leaching of the precious metals themselves, and/or PA0 (2) a process whereby a large proportion of the precious metal content of the feed material can be dissolved in a single leach using chlorine or nitric acid as the oxidant and hydrochloric acid as the dissolving medium and/or PA0 (3) a process whereby liquid recycle streams containing precious metals can be easily processed to recover the precious metal content without an intermediate zinc reduction step, by achieving reduction during base metal removal. PA0 (1) Concentrates having a high precious metal content (above about 50%) and which is essentially metallic in character (e.g. lead bullion residue after parting) may be treated as follows: PA0 (2) Treatment of concentrates having a medium precious metal content (between about 30% and 50%) with a high base metal content but where silica is present in low concentrations (less than 5%) or is preferably absent: This may be treated in similar fashion to the above except that often sufficient base metal oxides or sulphides are already present in the material to render the addition of further ferric oxide or ferrous sulphide unnecessary. Addition of some flux such as calcium oxide (CaO) may be helpful in rendering the aluminium oxide formed in the reaction soluble in the base metal leach. Certain matte leach residues and initial leach residues fall into this class of concentrate. PA0 (3) Material of medium or low precious metal content (precious metal content of less than 30%) with high base metal content and high silica content: The preferred technique here is to form an intermediate alloy of precious metals and iron which is then alloyed further with aluminium to form typically an alloy containing about 30% Fe, 30% Al, 30% precious metals and 10% other base metals such as Ni, Pb, Cu etc. This operation is typically but not necessarily carried out in an arc furnace as follows: PA0 (1) The aluminium and iron content of the alloy are leached together under highly reducing conditions, using weak acid to do the leaching. Any recycle streams that are acidic and contain precious metal values can be used in this step. Very effective cementation of the precious metal values has been experienced and the exhausted leach liquor will contain very low precious metal values. PA0 (2) The residue from the first leach is then subjected to a more intensive leach at a potential sufficient to dissolve copper. Many known methods are available for doing this and in particular the controlled potential leach system of Falconbridge (Journal of Metals, May 1975 p 6-9 "The recovery of nickel copper and a precious metal concentrate from high grade precious metal mattes" L. R. Hougan, H. Zacharissen) using chlorine gas as the oxidant would be applicable here. PA0 (3) The residue from the second leach is then leached in hydrochloric acid of commercial strength (about 10 Molar) using an oxidant which is preferably chlorine gas. Leaching has been found to be rapid and complete and the leach residue, if present, consists of insolubles such as silica and alumina with low precious metal content, typically less than 5000 ppm total precious metals. As an alternative to chlorine gas as the oxidant, nitric acid may be used thus forming the well known Aqua Regia.
Various recycle streams are also involved in the practical performance of this currently used procedure.
Thus a large number of operations is involved and a major proportion of the platinum metals is locked up in recycle streams. Many of the recycle streams are initially in the form of solutions; in order for the values contained in these streams to be recovered they must first be treated to render the precious metals insoluble. Commonly, reduction to metal using zinc as the reductant is employed, and this operation is time consuming, expensive and not entirely effective.
Furthermore it is commonly found with many precious metal bearing source materials that a large proportion of the base metal content cannot be effectively separated out before the platinum group metal leach by, for example, a milder leach such as sulphuric acid leaching. These base metals therefore contaminate the leach liquors containing the precious metals and render both refining operations and recycle considerably more difficult.